Arylhydrocarbon hydroxylase (AHH) activity was determined in cultured human epidermal and dermal cells with and without induction by benz[a]anthracene (BA). Large inter-individual variations were found in the basal and induced AHH activities and the induction ratio. The average basal AHH activity of 29 epidermal cultures in primary culture was 3.9 units (SD, 4.2; range, 0-16.3) (one unit was defined as 1 pmol of 3-hydroxybenzo[a]pyrene formed/mg protein/hr). AHH activity was induced time- and dose-dependently by BA. The average AHH activity induced by treatment with 13μM BA for 24hr was 97.4 units (SD, 69.4; range, 2.7-216.9). The induction ratio ranged from 0.6 to 262.0, but in the majority of cultures (15 of 27 cultures, 55%) it was less than 30. The mean induction ratio and SD was 51.1±63.0. The frequency distributions of basal AHH activity and the induction ratio were asymmetrical with a right tail. No correlation was found between the AHH activity or induction ratio and the age or sex of the donors or their residential district. The basal and induced AHH activities of dermal fibroblasts were both lower than those of epidermal cells; the average basal AHH level of 14 cultures was 1.5 units (SD, 1.4; range, 0-5.2), while the average induced AHH activity was 9.6 units (SD, 6.3; range 0-23.5). The induction ratio of dermal fibroblasts was lower than that of epidermal cells, the mean and SD being 7.5±7.4 (range, 1.9-27.0). There was no correlation between the AHH activities or induction ratios in epidermal and dermal cells isolated from the same donors.